Filled tutorial field in most relevent classes.

Added tutorial display in doc.

doc/classes/Node2D.xml

@@ -7,6 +7,7 @@
 		A 2D game object, with a position, rotation and scale. All 2D physics nodes and sprites inherit from Node2D. Use Node2D as a parent node to move, scale and rotate children in a 2D project. Also gives control on the node's render order.
 	</description>
 	<tutorials>
+	http://docs.godotengine.org/en/3.0/learning/features/2d/custom_drawing_in_2d.html
 	</tutorials>
 	<demos>
 	</demos>